Note: I took from the guide listed on the TrueAchievements site, posted by RuneSlayer4421
To get this achievement, you have to visit all of the 223 named Locations in the base game. There are many on each of the five planets plus seven located in space-based locations that are made available after unlocking their side mission.
You can check your progress for this by going to your map. Pressing [R] (on keyboard) or [Left Thumbstick] (on controller) will switch to show the progress details for the current Galaxy/Planet/Zone screen you are looking at.
For each of the planets, PowerPyx has done a pretty good location guide. The following are links to each individual planet's list.
Pandora - 78 Locations (1 in particular is very specific and easily missed, it is noted in the list)
http://www.powerpyx.com/borderlands-3-pandora-all-named-loca...Promethea - 43 Locations (2 in particular are very specific and easily missed, they are noted in the list)
http://www.powerpyx.com/borderlands-3-promethea-all-named-lo...Athenas - 5 Locations
http://www.powerpyx.com/borderlands-3-athenas-all-named-loca...Eden-6 - 63 Locations (2 in particular are very specific and easily missed, they are noted in the list)
http://www.powerpyx.com/borderlands-3-eden-6-all-named-locat...Nekrotafeyo - 27 Locations
http://www.powerpyx.com/borderlands-3-nekrotafeyo-all-named-...Also, each of the six Trials (and one Slaughter arena) give a new Zone in the galaxy that you need to travel to and walk around until the name pops up on screen. (So don't panic if you don't see the achievement pop after just finishing the planets!)
Note: This achievement has been known to be buggy, but the July 23rd update should have fixed that, including for those that have already unlocked all the Locations. (If you should already have unlocked this, make sure your game is updated then load in and explore at least one area, hopefully that should trigger it to check and unlock)